1. Field of the Invention
The present invention relates to an electrical joint forming member to be held between different conductive members included in a processing apparatus, and a plasma processing apparatus employing the electrical joint forming member.
2. Description of the Related Art
A semiconductor device fabricating procedure includes a process using a plasma produced by discharging a process gas for etching or film deposition. A plasma processing apparatus that carries out such a semiconductor device fabricating procedure includes a vacuum vessel and peripheral components formed by assembling a plurality of members to facilitate assembling and disassembling. Some joints of the members have a hermetic sealing function or a high-frequency current transmitting function. For example, the bear surfaces of aluminum members are brought into electrical contact with each other, and the aluminum members are fastened together with a plurality of bolts to ensure the transmission of high-frequency current through the members. A conductive cushioning member is placed in some joint to improve the electrical contact of the joint.
Referring to FIG. 15 showing an etching apparatus, which is a plasma processing apparatus, in a schematic sectional view, a support table serving also as a lower electrode 12 is mounted on the bottom wall of an aluminum vacuum vessel 1 through an insulating member 11. A shower head, through which a process gas is supplied into the vacuum vessel 1, serving also as an upper electrode 13 forms the top wall of the vacuum vessel 1. The inner surface of the side wall of the vacuum vessel 1 is covered with a cylindrical deposition shield 14 to prevent the deposition of matters on the inner surface of the side wall. The lower electrode 12 is connected to a high-frequency power source 15, and the vacuum vessel 1 is grounded. A high-frequency current flows from the upper electrode 13 through a surface part of the deposition shield 14 to the ground.
The conductive members including the deposition shield 14 and exposed to the atmosphere in the vacuum vessel 1 are required to be at the same potential (ground potential) to produce a uniform plasma. Therefore, the contact surfaces of the vacuum vessel 1 and the deposition shield 14 are pressed together with bolts, not shown, with cushioning members 16 held between the contact surfaces. The cushioning members 16 are, for example, stainless steel spiral seals such as disclosed in WO 00/75972 (pp. 11–12, FIGS. 8A and 8B).
As obvious from the results of experiments, which will be explained later, a contact load acting between contact surfaces of the spiral seal and the vacuum vessel is as low as about ⅕ of a contact load acting between contact surfaces of the deposition shield and the vacuum vessel when the deposition shield and the vacuum vessel are fastened together with bolts while the spiral seal held therebetween. Generally, the vacuum vessel is formed of aluminum. Measured contact resistance between an aluminum member and a stainless steel spiral seal determined through tests was considerably large when a contact load that was expected to act on the contact surfaces between the vacuum vessel and the stainless steel spiral fastened together with bolts acted on the contact surfaces between the aluminum member and a stainless steel spiral.
Recently, the frequency of high-frequency power used by the plasma processing apparatus is progressively increasing. Since the high-frequency current flows through a shallow surface part of a conductive member, the effect of the state of contact between conductive members on the surface potential of the conductive members is significant. Consequently, the effect of a process for processing a semiconductor wafer (hereinafter, referred to simply as “wafer”), such as an etching process, is sensitive to the state of contact. For example, in the plasma processing apparatus shown in FIG. 15, it is ideal that the entire surface of the deposition shield 14 is at the ground potential. However, if contact resistance between the conductive members is large, the potential of the surface of the deposition shield 14 becomes irregular and, consequently, a uniform plasma cannot be produced, the energy efficiency of high-frequency power decreases and etch rate decreases. As the minuteness of patterns of semiconductor integrated circuits increases progressively, the condition of a plasma process becomes more and more sensitive to the magnitude and distribution of the surface potential of conductive members. It is possible that the large contact resistance of a joint provided with a stainless steel spiral seal reduces yield.
Moreover, a high torque is applied to the bolts for fastening the members together. Therefore, the conductive members deform by the repetition of assembly and disassembly for maintenance and long period of use, and face pressure acting on the conductive members changes. Since a processing apparatus for processing large-diameter wafers includes large component parts which thus deform largely, causing in-plane uniformity of plasma-processed wafers and etch rate to change. Increase in the number of bolts fastening together members with a spiral seal held between the members to increase face pressure acting on the members will adversely affect the maintenance of the processing system.
A cushioning member disclosed in JP8-120469A (Paragraph 0016) is formed by covering the surface of a rubber tube with a stainless steel mesh or a copper mesh. The cushioning member provided with the stainless steel mesh also cannot ensure satisfactory electrical contact. Although the cushioning member provided with the copper mesh has a low contact resistance because copper has a small specific resistance, the copper mesh can cause the metal-contamination of semiconductor integrated circuits and hence the cushioning member provided with the copper mesh cannot be practically applied to semiconductor device fabricating apparatuses.